Heterogeneous silica carbon black-filled rubber compound

ABSTRACT

The present invention relates to a heterogeneous silica/carbon black-filled rubber compound and a process for making the same. The rubber compound has improved (lower) electrical resistivity compared to homogeneous silica-filled rubber compounds. The process comprises 
     (a) intimately dispersing from 30 to 60 phr of a silica filler and 5 to 80 phr of a carbon black in a first rubber to form a first nonproductive compound; 
     (b) intimately dispersing curatives in said first nonproductive compound to form a first productive compound; 
     (c) separately intimately dispersing from 20 to 100 phr of a silica filler in a second rubber, which is different from said first rubber, to form a second nonproductive compound; and 
     (d) intimately dispersing curatives in said second nonproductive compound to form a second productive compound; and 
     (e) mixing said first productive compound with said second productive compound to form a heterogeneous silica/carbon black-filled rubber compound.

This is a divisional of application Ser. No. 09/153,438, filed on Sep.15, 1998, now U.S. Pat. No. 6,121,367 presently pending which claimspriority to U.S. Provisional Application Serial No. 60/059,671, filedSep. 18, 1997.

BACKGROUND OF THE INVENTION

There has recently developed trends in some tire markets forsilica-based tread markets. With this trend, increasing levels of silicahave been used with concomitant reductions in the levels of carbonblack. As one increases the level of silica in a rubber with decreasinglevels of carbon black, the electrical resistivity of the rubberincreases. This is undesirable when the rubber is used in a tire due tothe potential building of static electricity in the vehicle on which thetires are mounted. While it is described in some instances to use silicafillers, it is also desirable to avoid the potential problems when thecarbon black levels are decreased.

SUMMARY OF THE INVENTION

The present invention relates to heterogeneous silica/carbonblack-filled rubber compounds and a process for making such compounds.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed a process for the production of a heterogeneoussilica/carbon black-filled rubber compound comprising

(a) intimately dispersing from 30 to 60 phr of a silica filler and 5 to80 phr of a carbon black in a first rubber to form a first nonproductivecompound;

(b) intimately dispersing curatives in said first nonproductive compoundto form a first productive compound;

(c) separately intimately dispersing from 20 to 100 phr of a silicafiller in a second rubber, which is different from said first rubber, toform a second nonproductive compound;

(d) intimately dispersing curatives in said second nonproductivecompound to form a second productive compound; and

(e) mixing said first productive compound with said second productivecompound to form a heterogeneous silica/carbon black-filled rubbercompound.

There is also disclosed a heterogenous silica black-filled rubbercompound prepared by a process comprising

(a) intimately dispersing from 30 to 60 phr of a silica filler and 5 to80 phr of a carbon black in a first rubber to form a first nonproductivecompound;

(b) intimately dispersing curatives in said first nonproductive compoundto form a first productive compound;

(c) separately intimately dispersing from 20 to 100 phr of a silicafiller in a second rubber, which is different from said first rubber, toform a second nonproductive compound;

(d) intimately dispersing curatives in said second nonproductivecompound to form a second productive compound; and

(e) mixing said first productive compound with said second productivecompound to form a heterogeneous silica/carbon black-filled rubbercompound.

The present invention relates to heterogeneous silica/carbonblack-filled rubber compound. Heterogeneous as used herein meanscontaining dissimilar ingredients and/or levels of ingredients. Morespecifically, the term means a rubber compound containing the selectivedispersion of silica and carbon black in a first rubber and silica in asecond rubber. Thereafter, upon adding curatives to each rubber compoundand the subsequent mixing of the two productives, there is a higherconcentration of the carbon in one phase, resulting in improvedconductivity of the overall rubber compound and more optimum curing ofeach of the two phases.

One critical aspect of the present invention is that the first rubberand second rubber are different. Even though the two rubbers aredifferent, each of the two rubbers may be selected from the same groupof rubbers. The first rubber and second rubber may be selected from thegroup consisting of emulsion polymerized styrene/butadiene copolymers,solution polymerized styrene/butadiene copolymers, natural rubber, cis1,4-polybutadiene, synthetic cis 1,4-polyisoprene, styrene/isoprenecopolymers, 3,4-polyisoprene, isoprene/butadiene copolymers, mediumvinyl polybutadiene (20 percent to 60 percent by weight of vinyl units),styrene/isoprene/butadiene terpolymers, butyl rubber, polychloroprene,acrylonitrile/butadiene copolymers and ethylene/propylene/dieneterpolymers and mixtures thereof.

In accordance with a preferred embodiment of the present invention, thefirst rubber (used to make the first nonproductive) and the secondrubber (used to make the second nonproductive) have different glasstransition temperatures (Tg). The term “Tg” refers to the glasstransition temperature of the identified rubber and is suitablydetermined by a differential scanning calorimeter at a rate of 10° C.per minute. In a particularly preferred embodiment, the Tg of the firstrubber is higher than the Tg of the second rubber. For example, the Tgof the first rubber may range from −50° C. to 0° C. (high Tg rubber) andthe Tg of the second rubber may range from −100° C. to −51° C. (low Tgrubber). In accordance with this embodiment, the difference between theTg of the first and second rubber generally ranges from about 60° C. to0° C.

Representative examples of high Tg rubbers include 3,4 polyisoprenewhich typically contains about 65 weight percent 3,4-isoprene units andhas a Tg of about −16° C. Another example of a high Tg rubber is asolution-polymerized styrene/butadiene copolymer rubber containing 12weight percent styrene, a vinyl content of about 40 weight percent, a Tgof −45° C. and a Mooney viscosity (ML 1+4) at 100° C. of 90. Anotherhigh Tg rubber is a styrene/isoprene/butadiene terpolymer rubbercontaining 20 weight percent styrene, 40 weight percent isoprene and 40percent butadiene, a Tg of −42° C. and a Mooney viscosity at 100° C. of90. Yet another high Tg rubber is a high cis 1,4-polybutadiene rubbercharacterized by the weight percent of 1,4-bonds of at least 9 percent.Another high Tg rubber is a solution-polymerized medium cis1,4-polybutadiene 40 to 60 weight percent of the units of a vinyl1,2-structure and 35 to 45 weight percent of its units of a cis1,4-structure. Such solution-polymerized polybutadiene has a Tg of −65°C and Mooney viscosity (ML 1+4) at 100° C. of about 44. Additionalexamples of high Tg rubbers are emulsion-polymerized styrene/butadienecopolymer rubber characterized by a weight percent of from 23.5 to 40weight percent styrene. For example, an emulsion-polymerizedstyrene/butadiene copolymer rubber having 23.5 weight percent styrenetypically has a Tg of about −55° C. An emulsion-polymerizedstyrene/butadiene copolymer rubber having 40 weight percent styrenetypically has a Tg of about −32° C. to −35° C. The preferred high Tgrubber will depend on the application of the rubber compound of thepresent invention.

Representative examples of the low Tg rubber include polybutadienerubber having 95 weight percent or more cis 1,4-structure, a Tg of from−95° C. to −105° C. and a Mooney viscosity (ML 1+4) at 100° C. of 5 from30 to 100. Another example of a low Tg rubber is an isoprene/butadienecopolymer rubber prepared by neodymium catalysis and characterized byhaving an isoprene content of about 20 weight percent, a Tg of about−90° C. and a Mooney viscosity (ML 1+4) at 100° C. of 82. Yet anotherexample is an isoprene/butadiene copolymer rubber prepared by neodymiumcatalysis and characterized by having an isoprene content of about 10weight percent, a Tg of about −98° C. and a Mooney viscosity (ML 1+4) at100° C. of 82. Other examples of suitable rubbers aresolution-polymerized styrene/butadiene copolymer rubbers containing upto 10 weight percent of styrene. Such styrene/butadiene copolymersexhibit a Tg of from −93° C. to −80° C. and Mooney viscosities (ML 1+4)at 100° C. from 30 to 100. The preferred low Tg rubber will depend onthe application of the rubber compound of the present invention.

Another example is cis 1,4-polyisoprene. The cis 1,4-polyisoprene rubberincludes both natural and synthetic rubbers. The cis 1,4-polyisoprenerubber, natural or synthetic, typically has a cis 1,4-content of about96 to about 99 weight percent. Synthetic cis 1,4-polyisoprene generallyhas a Tg of about −65° C. Natural rubber typically has a Tg of about−65° C. Typical Mooney viscosities (ML 1+4) at 100° C. for synthesis cis1,4-polyisoprene and natural rubber range from 30 to 100. Another low Tgrubber is an isoprene/butadiene copolymer rubber prepared by neodymiumcatalysis characterized by an isoprene content of about 70 weightpercent, a Tg of about −79° C. and a Mooney viscosity (ML 1+4) at 100 °C. of 76. Another low Tg rubber is solution-polymerizedstyrene/butadiene copolymer rubber having a styrene content of about 18weight percent, a vinyl content of about 10 weight percent, a Tg ofabout −78° C. and a Mooney viscosity (ML 1+4) at 100° C. of 85.

In addition to the first rubber in the first nonproductive, anotherrubber may also be present. In those instances where two or more rubbersare used in the first nonproductive, the first rubber should be used inan amount ranging from about 30 to 80 parts by weight per 100 parts byweight of the total rubber (phr), based on the first nonproductives.Preferably, the first rubber should be present in the firstnonproductive in an amount ranging from about 40 to 60 phr. Therefore,if one or more rubbers are used in addition to the first rubber to makethe first nonproductive, the total level of such rubbers(s) in theoverall heterogenous blend which are derived from the firstnonproductive should range from 20 to 80 phr, with a range of from 40 to60 phr being preferred.

In those instances where another rubber or rubbers are used in the firstnonproductive, it is preferred to observe the same guidelines as toselection of the first rubber; namely, with respect to the described Tgguidelines.

Similarly, the second rubber in the second nonproductive, another rubbermay also be present. In those instances where two or more rubbers areused in the second nonproductive, the second rubber should be used in anamount ranging from about 20 to 70 parts by weight per 100 parts byweight of total rubber (phr), based on the second nonproductive.Preferably, the second rubber should be present in the secondnonproductive in an amount ranging from about 40 to 60 phr. Therefore,if one or more rubbers are used in addition to the second rubber to makethe second in the overall heterogeneous blend which are derived from thesecond nonproductive, the total level of such rubbers(s) should rangefrom 20 to 80 phr, with a range of from 40 to 60 phr being preferred.

In those instances where another rubber or rubbers are used in thesecond nonproductive, it is preferred to observe the same guidelines asto selection of the second rubber; namely, with respect to the describedTg guidelines.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300.

The silica might be expected to have an average ultimate particle size,for example, in the range of 0.01 to 0.05 micron as determined by theelectron microscope, although the silica particles may be even smaller,or possibly larger, in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3, etc. The Rhone-Poulenc Z1165MP silica is currently preferred.

The silica is added to both the first and second nonproductivecompounds. The level of silica that is present in the firstnonproductive may range from about 30 to 60 phr, based on the totalrubber in the first nonproductive. Preferably, the level of silica thatis added to the first nonproductive ranges from 40 to 60 phr. The levelof silica that is present in the second nonproductive may range fromabout 20 to 100 phr, based on the total rubber in the secondnonproductive. Preferably, the level of silica that is added to thesecond nonproductive ranges from 60 to 80 phr.

The silica is intimately dispersed in the first nonproductive compoundand the second nonproductive compound. The mixing may be accomplished bymethods known to those skilled in the rubber mixing art. For example,fixed and variable speed mixers or Banburys™ may be used. The firstrubber and silica as well as the second rubber and silicas are mixed ina nonproductive mix stage. The silica and first rubber as well as thesilica and second rubber are mixed for a time and temperature tointimately disperse the silica. For example, mixing at a rubbertemperature from 130 to 180° C. for a period of from 10 seconds to 20minutes.

In addition to the first rubber or second rubber (as well as theoptional rubbers as described above) and silica, a silica coupling agentmay be present in the first nonproductive compound and secondnonproductive compound. The silica coupling agent is used to promote theinteraction of the silica and the rubber. Various known silica couplersmay be used.

One example of a silica coupler is a sulfur containing organosiliconcompound. Examples of sulfur containing organosilicon compounds are ofthe formula:

Z—Alk—S_(n)—Alk—Z

in which Z is selected from the group consisting of

where

R¹ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl;

R² is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbonatoms;

Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is aninteger of 2 to 8.

Specific examples of sulfur containing organosilicon compounds which maybe used in accordance with the present invention include:3,3′-bis(trimethoxysilylpropyl)disulfide,3,3′-bis(triethoxysilylpropyl)tetrasulfide,3,3′-bis(triethoxysilylpropyl)octasulfide,3,3′-bis(trimethoxysilylpropyl)tetrasulfide,2,2′-bis(triethoxysilylethyl)tetrasulfide,3,3′-bis(trimethoxysilylpropyl)trisulfide,3,3′-bis(triethoxysilylpropyl)trisulfide,3,3′-bis(tributoxysilylpropyl)disulfide,3,3′-bis(trimethoxysilylpropyl)hexasulfide,3,3′-bis(trimethoxysilylpropyl)octasulfide,3,3′-bis(trioctoxysilylpropyl)tetrasulfide,3,3′-bis(trihexoxysilylpropyl)disulfide,3,3′-bis(tri-2″-ethylhexoxysilylpropyl)trisulfide,3,3′-bis(triisooctoxysilylpropyl)tetrasulfide,3,3′-bis(tri-t-butoxysilylpropyl)disulfide, 2,2′-bis(methoxy diethoxysilyl ethyl)tetrasulfide, 2,2′-bis(tripropoxysilylethyl)pentasulfide,3,3′-bis(tricyclonexoxysilylpropyl)tetrasulfide,3,3′-bis(tricyclopentoxysilylpropyl)trisulfide,2,2′-bis(tri-2″-methylcyclohexoxysilylethyl)tetrasulfide,bis(trimethoxysilylmethyl)tetrasulfide,3-methoxy ethoxy propoxysilyl 3′-diethoxybutoxysilylpropyltetrasulfide,2,2′-bis(dimethyl methoxysilylethyl)disulfide, 2,2′-bis(dimethylsec.butoxysilylethyl)trisulfide, 3,3′-bis(methylbutylethoxysilylpropyl)tetrasulfide, 3,3′-bis(dit-butylmethoxysilylpropyl)tetrasulfide, 2,2′-bis(phenyl methylmethoxysilylethyl)trisulfide, 3,3′-bis(diphenylisopropoxysilylpropyl)tetrasulfide, 3,3′-bis(diphenylcyclohexoxysilylpropyl)disulfide, 3,3′-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2′-bis(methyldimethoxysilylethyl) trisulfide, 2,2′-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3′-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3′-bis(ethyl di-sec.butoxysilylpropyl)disulfide, 3,3′-bis(propyldiethoxysilylpropyl)disulfide, 3,3′-bis(butyldimethoxysilylpropyl)trisulfide, 3,3′-bis(phenyldimethoxysilylpropyl)tetrasulfide, 3-phenyl ethoxybutoxysilyl3′-trimethoxysilylpropyl tetrasulfide, 4,4′-bis(trimethoxysilylbutyl)tetrasulfide, 6,6′-bis(triethoxysilylhexyl) tetrasulfide,12,12′-bis(triisopropoxysilyl dodecyl) disulfide,18,18′-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18′-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4′-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4′-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5′-bis(dimethoxymethylsilylpentyl) trisulfide,3,3′-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide,3,3′-bis(dimethoxyphenylsilyl-2-methylpropyl)disulfide.

The preferred sulfur containing organosilicon compounds are the3,3′-bis(trimethoxy or triethoxy silylpropyl)sulfides. The mostpreferred compound is 3,3′-bis(triethoxysilylpropyl)tetrasulfide.Therefore, as to the above formula, preferably Z is

where R² is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms beingparticularly preferred; Alk is a divalent hydrocarbon of 2 to 4 carbonatoms with 3 carbon atoms being particularly preferred; and n is aninteger of from 2 to 5 with 4 being particularly preferred.

The amount of the sulfur containing organosilicon compound in a rubbercomposition will vary depending on the level of silica that is used.Generally speaking, the amount of the organosilicon compound will rangefrom 0.5 to 50 phr. Preferably, the amount will range from 1.5 to 8 phr.Depending on the desired properties, the weight ratio of the sulfurcontaining organosilicon compound to silica may vary. Generallyspeaking, the weight ratio will range from 1:100 to 1:5. Preferably, theweight ratio will range from 1:20 to 1:10.

In the second critical step of the present invention, carbon black isintimately dispersed along with the silica in the first rubber, which isdifferent from the first rubber, to form the first nonproductivecompound. Typical amounts of reinforcing-type carbon black(s), rangefrom 5 to 80 phr. Preferably, the carbon black level ranges from 20 to50 phr. Representative of the carbon blacks which may be used in thepresent invention include those known to those skilled in the art underthe ASTM designations N110, N121, N220, N231, N234, N242, N293, N299,S315, N326, N330, N332, N339, N343, N347, N351, N358, N375 and mixturesthereof. Carbon fibers may also be added to the rubber compound.

The weight ratio of silica to carbon black in the overall heterogeneoussilica carbon black-filled rubber compound may vary. For example, theweight ratio of silica to carbon black may be as low as 1:5 to 30:1.Preferably, the weight ratio of silica to carbon black ranges from 2:1to 6:1.

Essentially all of the carbon black that is used is added to the firstnonproductive, with the exception of those rubber chemicals, such assulfur containing organosilicons which are dispersed on carbon black andadded at a different stage of mixing or to the second nonproductive.Preferably from 80 to 100 weight percent of the total carbon black isadded to the first rubber not including the carbon black used as acarrier.

Both the first nonproductive compound and second nonproductive compoundmay contain various commonly used additive materials such as, forexample, processing additives such as oils, resins including tackifyingresins and plasticizers, pigments, fatty acid, zinc oxide, waxes,antioxidants and antiozonants and peptizing agents. Depending on theintended use of the heterogeneous silica/carbon black-filled rubbercompound, the additives mentioned above are selected and commonly usedin conventional amounts. Typical amounts of tackifier resins, if used,comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr.Typical amounts of processing aids comprise about 1 to about 50 phr.Such processing aids can include, for example, aromatic, napthenic,and/or paraffinic processing oils. Typical amounts of antioxidantscomprise about 1 to about 5 phr. Representative antioxidants may be, forexample, diphenyl-p-phenylenediamine and others, such as, for example,those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-346.Typical amounts of antiozonants comprise about 1 to 5 phr. Typicalamounts of fatty acids, if used, which can include stearic acid compriseabout 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2to about 5 phr. Typical amounts of waxes comprise about 1 to about 5phr. Often microcrystalline waxes are used. Typical amounts of peptizerscomprise about 0.1 to about 1 phr. Typical peptizers may be, forexample, pentachlorothiophenol and dibenzamidodiphenyl disulfide.

The above additives may be mixed in the first nonproductive compound orsecond nonproductive compound in any ratio and due to the differentcompounds will be used at different levels.

The above conventional ingredients may also be added to subsequent mixesincluding the productive step.

Once the first nonproductive compound and second nonproductive has beenprepared, the two productive compounds are prepared separately. Theproductive step involves a mixing stage where the curatives are added.For example, sulfur donors or sulfur vulcanizing agents includeelemental sulfur (free sulfur), an amine disulfide, polymericpolysulfide and sulfur olefin additives. Preferably, the sulfurvulcanizing agent is elemental sulfur. The sulfur vulcanizing agent maybe used in an amount ranging from 0.5 to 8 phr, with a range of from 0.5to 4 being preferred. Accelerators are used to control the time and/ortemperature required for vulcanization and to improve the properties ofthe vulcanizate. In one embodiment, a single accelerator system may beused, i.e., primary accelerator. The primary accelerator(s) may be usedin total amounts ranging from about 0.5 to about 4, preferably about 0.8to about 1.5, phr. In another embodiment, combinations of a primary anda secondary accelerator might be used with the secondary acceleratorbeing used in smaller amounts (of about 0.05 to about 3 phr) in order toactivate and to improve the properties of the vulcanizate. Combinationsof these accelerators might be expected to produce a synergistic effecton the final properties and are somewhat better than those produced byuse of either accelerator alone. In addition, delayed actionaccelerators may be used which are not affected by normal processingtemperatures but produce a satisfactory cure at ordinary vulcanizationtemperatures. Vulcanization retarders might also be used. Suitable typesof accelerators that may be used in the present invention are amines,disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides,dithiocarbamates and xanthates. Preferably, the primary accelerator is asulfenamide. If a second accelerator is used, the secondary acceleratoris preferably a guanidine, dithiocarbamate or thiuram compound.

The curatives that are added to make the first productive compound maybe the same or different and/or used at different levels. Preferably,each productive compound has a curative particularly designed for itbased on the rubbers used as well as other ingredients present.

The curatives, including sulfur vulcanizing agents, are separately mixedwith the first and second nonproductive compounds. Mixing of anonproductive compound with curatives is conventionally called“productive” mix stage. Productive mixing typically occurs at atemperature, or ultimate temperature lower than the mix temperature(s)of the preceding nonproductive stage(s) and always below the subsequentcure temperatures. Typical mixing of the productive compound is at arubber temperature ranging from 90 to 110° C. for a period of 30 secondsto 3 minutes.

After the first and second productive compounds have been prepared, thetwo productive stocks are mixed together. It is preferred to have aslittle mixing as possible to maintain the heterogeneous nature of theoverall rubber. As can be appreciated, the more mixing the morehomogeneous the rubber will be, resulting in each of the productivecompounds losing their identity to the other.

Generally speaking, the two productive stocks are mixed in conventionalequipment such as an extruder, mill or Banbury.

The overall heterogeneous rubber may contain various weight percentagesof the first productive rubber and second productive rubber. Generallyspeaking, the heterogeneous rubber contains from about 40 to 80 parts byweight of the first productive rubber and from 60 to 20 parts by weightof the second productive rubber. Preferably, the heterogeneous rubbercontains from about 60 to 80 parts by weight of the first productiverubber and from 40 to 20 parts by weight of the second productiverubber.

Vulcanization of the rubber composition of the present invention isgenerally carried out at conventional temperatures ranging from 130° C.to 200° C. Preferably, the vulcanization is conducted at temperaturesranging from 140° C. to 160° C. Any of the usual vulcanization processesmay be used such as heating in a press or mold, heating with superheatedsteam or hot air or in a salt bath.

Upon vulcanization of the sulfur vulcanized composition, the rubbercomposition of this invention can be used for various purposes. Forexample, the sulfur vulcanized rubber composition may be in the form ofa pneumatic tire, belt, hose, air spring, shoe product and motor mount.In case of a tire, it can be used for various tire components. Suchtires can be built, shaped, molded and cured by various methods whichare known and will be readily apparent to those having skill in suchart. Preferably, the rubber composition is used in the tread of a tire.As can be appreciated, the tire may be a passenger tire, aircraft tire,truck tire and the like. Preferably, the tire is a passenger tire. Thetire may also be a radial or bias, with a radial tire being preferred.

The invention may be better understood by reference to the followingexamples in which the parts and percentages are by weight unlessotherwise indicated.

The following examples are presented in order to illustrate but notlimit the present invention.

The following tables report conductivity properties that were determinedfrom the rubber stocks that were prepared.

EXAMPLE 1

Tables 1 and 2 compare standard “all-in” mixed compound (Control) versusa phase-mixed compound (Sample 3) which are of the same formulations(61.9/13.3 phr silica/carbon black). All of the ingredients in theControl were mixed in one nonproductive step and one productive. InSamples 1 and 2 involved, two separate nonproductive steps followed bytwo separate production steps were used. In each nonproductive mixingstep, a Kobe™ BB-2 mixer was used. The nonproductive step of allcompounds containing silica (Standard and Nonproductives A and B)utilized variable speed mixing.

For Control, the rubber was loaded and mixed at 60 RPM. After 30seconds, all of the silica coupler and one-half of the silica filler wasadded. After 60 seconds, the remaining silica, carbon black and otheradditives were loaded. Once the rubber temperature reached 160° C., therotor speed was adjusted to maintain 160° C. for 2 minutes. Thenonproductive compound was then discharged.

For the Nonproductive A step in Sample 1, the above procedure wasrepeated except no carbon black was added.

For the Nonproductive B step in Sample 2, the mixing was at 55 rpm. Therubber, carbon black, silica and remaining additives were loaded. After150 seconds, the rubber temperature was approximately 170° C., themixing was stopped and its contents removed.

For the Productive step for Control, one-half of the amount of thenonproductive mix was loaded along with the curatives followed by theremaining one-half of the nonproductive. Mixing was then commenced forapproximately 2 minutes at 35 rpm. The rubber temperature reachedapproximately 90 to 100° C. and the load was discharged.

For the Productive A step for Sample 1, half the Nonproductive A wasloaded along with the curatives followed by the remaining half ofNonproductive A. Similarly, for the Productive B step for Sample 2, halfthe Nonproductive B was loaded along with the curatives followed by theremaining half of Nonproductive B. Mixing was then commenced for bothProductives approximately 2 minutes at 35 rpm. The rubber temperaturereached approximately 90 to 100° C. and the loads was discharged. ForSample 3 (phase-mixed), 30 parts by weight of Productive A (Sample 1)and 70 parts by weight of Productive B (Sample 2) were then combined andmixed for approximately 2 minutes at 30 rpm. The rubber temperaturereached 90° C. and the load was discharged.

TABLE 1 STANDARD VS PHASE-MIXED COMPOUNDS Sample Sample Sample Control 12 Sample 3 Nonpro- Nonpro- Mix Procedure Standard Mixed ductive ductive30:70 Component Nonproductive A B Phase Mixed Natural Rubber¹ 10 33.33 010 Polybutadiene² 20 66.67 0 20 IBR³ 45 0 64.28 45 SBR⁴ 34.38 0 49.1134.38 Oil 14 7 17 14 Carbon Black⁵ 13.3 0 19 13.3 Silica⁶ 61.9 50 6761.9 Silica Coupler⁷ 9.5 8 10.14 9.5 Pro- Pro- 30:70 Standard Mixedductive ductive Phase Mixed Productive A B Productive ZnO 2.50 2.50 2.502.50 Sulfur 1.40 1.40 1.40 1.40 Accelerator⁸ 1.65 1.43 1.74 1.65Accelerator⁹ 2.00 1.00 2.43 2.00 Stearic Acid 3.00 3.00 3.00 3.00¹SMR-20 ²A solution-polymerized polybutadiene rubber having 96 weightpercent cis 1,4-structure, a Tg of −104° C. and cis commerciallyavailable from The Goodyear Tire & Rubber Company as Budene ® 1207. ³Anisoprene-butadiene copolymer prepared via solution polymerization. TheIBR had a 50 percent by weight isoprene content, a 50 percent by weightbutadiene content, a cis-1,4-bond content of 48 percent of −42° C. ⁴Anemulsion-polymerized styrene/butadiene rubber having 40 percent byweight bound styrene, 37.5 phr of oil and a Tg of −32° C. ⁵N299 Black⁶Zeosil 1165 MP commercially available from Rhone Poulenc⁷3,3′-bis(triethoxysilylpropoyl) tetrasulfide on a carbon black carrier(50%—50% by weight) commercially obtained from Degussa under thecommercial designation X50S ⁸N-cyclohexyl benzothiazole-2-sulfenamide⁹diphenyl guanidine

Sample Control Sample 1 Sample 2 Sample 3 Mix Procedure Standard MixProductive Productive 30:70 (A:B) Component Control A B Phase MixedElectrical 1.2 E 14 2.7 E 16 5.3 E 5 2.9 E 7 Surface Resistivity (Ohms)

What is claimed is:
 1. A heterogeneous silica/carbon black-filled rubbercompound prepared by a process comprising (a) intimately dispersing from30 to 60 phr of a silica filler and 5 to 80 phr of a carbon black in afirst rubber to form a first nonproductive compound; (b) intimatelydispersing curatives in said first nonproductive compound to form afirst productive compound; (c) separately and intimately dispersing from20 to 100 phr of a silica filler in a second rubber, which is differentfrom said first rubber to form a second nonproductive compound; (d)intimately dispersing curatives in said second nonproductive compound toform a second productive compound; and (e) mixing said first productivecompound with said second productive compound to form a heterogeneoussilica/carbon black-filled rubber compound.
 2. The compound of claim 1wherein said first rubber and said second rubber are selected from thegroup consisting of emulsion polymerized styrene/butadiene copolymers,solution polymerized styrene/butadiene copolymers, natural rubber, cis1,4-polybutadiene, styrene isoprene copolymers, isoprene/butadienecopolymers, medium vinyl polybutadiene, styrene/isoprene/butadieneterpolymers, butyl rubber, polychloroprene, acrylonitrile/butadienecopolymers and ethylene/propylene/diene terpolymers and mixturesthereof.
 3. The compound of claim 1 wherein said first rubber has a Tghigher than the Tg of said second rubber.
 4. The compound of claim 1wherein said first rubber is selected from the group consisting ofsolution polymerized styrene/butadiene copolymers, isoprene/butadienecopolymers and mixtures thereof.
 5. The compound of claim 1 wherein saidsecond rubber is selected from the group consisting of cis1,4-polybutadiene and natural rubber.
 6. The compound of claim 1 whereinsaid curatives include a sulfur vulcanization agent and at least oneaccelerator.
 7. The compound of claim 1 wherein from 40 to 60 phr of asilica filler and from 20 to 50 phr of a carbon black are intimatelydispersed with said first rubber compound to form said firstnonproductive compound.
 8. The compound of claim 1 wherein from 60 to 80phr of silica filler is intimately dispersed with said second rubbercompound to form said second nonproductive compound.
 9. The compositionof claim 1 wherein a silica coupling agent is intimately dispersed withsaid first and second nonproductive compounds.
 10. The compound of claim1 wherein said silica coupler is a sulfur containing organosiliconmaterial.
 11. The compound of claim 10 wherein said sulfur containingorganosilicon compound is of the formula: Z—Alk—S_(n)—Alk—Z in which Zis selected from the group consisting of

where R¹ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl;R² is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbonatoms; Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is aninteger of 2 to
 8. 12. The compound of claim 1 wherein said first andsecond nonproductive compounds are separately mixed at a rubbertemperature of from 130° C. to 180° C. for a period of from 10 secondsto 20 minutes.
 13. The compound of claim 1 wherein said heterogeneoussilica/carbon black-filled rubber compound is derived from 60 to 80parts by weight of said first productive compound and from 60 to 20parts by weight of said second productive compound.
 14. The compound ofclaim 1 wherein said heterogeneous silica/carbon black-filled rubbercompound is derived from 60 to 80 parts by weight of said firstproductive compound and from 40 to 20 parts by weight of said secondproductive compound.
 15. The compound of claim 1 wherein an additionalrubber is present to form said first nonproductive compound.
 16. Thecompound of claim 1 wherein an additional rubber is present to form saidsecond nonproductive compound.
 17. The compound of claim 1 wherein saidcarbon black is selected from the group consisting of N110, N121, N220,N231, N234, N242, N293, N299, S315, N326, N330, N332, N339, N343, N347,N351, N358, N375 and mixtures thereof.
 18. The compound of claim 1 whichis vulcanized at a temperature ranging from 100° C. to 200° C.
 19. Thecompound of claim 18 which is in the form selected from the groupconsisting of a pneumatic tire, belt, hose, air spring, shoe product andmotor mount.
 20. A pneumatic tire having a tread comprised of thecomposition of claim 19.